(1) Field of the Invention
The present invention relates to a membrane for the separation of a water-alcohol mixed liquid and a process for the preparation thereof. More particularly, the present invention relates to a membrane suitable for separating a water-alcohol mixed liquid by the pervaporation method and a process for the preparation of this separating membrane.
(2) Description of the Related Art
Distillation has been mainly utilized for the separation of a water-alcohol mixed liquid, but this distillation process is defective in that the energy cost is large and since a water-alcohol mixed liquid often has an azeotropic composition, the separation is insufficient.
Accordingly, various separating methods using a membrane have attracted attention, and various membranes for use in separating water-alcohol mixed liquids by the pervaporation method (a liquid mixture is supplied into the primary side, the pressure in the secondary side separated from the primary side through a separating membrane is reduced, and a certain component in the liquid mixture is selectively diffused into the secondary side and recovered in the form of a vapor in the secondary side) have been reported. For example, the specification of U.S. Pat. No. 2,953,502 discloses the separation of an azeotropic liquid mixture of water and ethyl alcohol or the like by using a cellulose acetate membrane or the like as the separating membrane, J. Polym. Sci., Symposium No. 41, pages 145-153 (1973) discloses the separation of a water-methanol mixed liquid by using a separating membrane of cellophane, and J. Applied Polym. Sci., vol. 26, pages 3223-3243 (1981) discloses the separation of a water-methanol mixed liquid by using a separating membrane of grafted polyvinyl alcohol. However, the separation of water-alcohol mixed liquids by the prevaporation method using these separating membranes is defective in that the ratio of separation by one permeation of the water-alcohol mixed liquid through the separating membrane, that is, the separation coefficient, is small and the quantity of permeation through the separating membrane is small, and therefore, the separation performances are insufficient.
As the means for solving these problems of the conventional separating membranes to be used in the pervaporation method, a separating membrane composed of chitosan, which is a natural cationic polymer, has recently been proposed. For example, Japanese Patent Application Laid-Open Specification No. 56-190706 (Japanese Patent Publication No. 63-39281) discloses a process in which a liquid mixture of water and ethanol or the like is separated by the pervaporation method using a chitosan type membrane, Japanese Patent Application Laid-Open Specification No. 62-4407 discloses a separating membrane for the pervaporation, which is composed of a polymer obtained by polymerizing a vinyl monomer to a chitosan compound, and Japanese Patent Application Laid-Open Specification No. 63-7403 discloses a separating membrane for the pervaporation which is composed of a chitosan salt or chitosan derivative salt. Separating membranes composed of chitosan compounds exert considerable separation performances in the separation of water-alcohol mixed liquids by the pervaporation, but no satisfactory results can be obtained on an industrial scale and especially, it is desired that the separation coefficient will be further improved. The separation coefficient (.alpha.) referred to herein is represented by the following equation. ##EQU1## As is apparent from the above equation, the separation coefficient is the ratio of the weight ratio (W.sub.A /W.sub.B) between the components A and B in the permeated liquid after the permeation to the weight ratio (W.sub.A /W.sub.B) in the supplied liquid before the permeation. When water is regarded as the base (component A), if .alpha. is larger than 1, water is selectively permeated, and the larger is the value .alpha., the higher is the selective permeability.